Potentially lethal force may be being used to remove detainees from the UK, MPs have warned.
Unauthorised and potentially fatal control and restraint techniques, including holding the detainee's head down, may be used by staff on removal flights, despite bosses insisting such moves are never used, a report said.
The inquiry, by the Commons Home Affairs Select Committee, comes after the death of a man who died as he was being deported from the UK.
Angolan national Jimmy Mubenga, 46, collapsed on a plane at Heathrow Airport while being removed on October 12, 2010.
The MPs did not comment on Mr Mubenga's case, which is still being investigated by police.
But they said: "We are not persuaded that head-down restraint positions are never used, even though they are not authorised."
They called for the Home Office to issue urgent guidance to all staff involved in enforced removals "about the danger of seated restraint techniques in which the subject is bent forwards".
"Reports of head-down restraint positions are troubling in the light of recent evidence which shows that the prolonged use of such positions might carry a risk of death."
Both the UK Border Agency (UKBA) and contractors, including private security firm G4S, denied that any techniques to hold the neck or keep the head down were used.
But a report to the UKBA by Baroness O'Loan on Outsourcing Abuse "describes several incidents in which detainees claim to have been restrained with their heads held down".
"It is difficult to believe that all these accounts are complete fabrications," the MPs said.
They called for the UKBA to strengthen its procedures so that its staff feel they are entitled and expected to challenge any poor conduct on the part of contractors.
The committee also called for an end to the practice of taking detainees from removal centres to airports as "reserves", in case another detainee is taken off the flight at the last minute.
Keith Vaz, the committee's chairman, said: "People who are not entitled to remain in the country must be removed and there may be occasions when it is necessary to use physical force, but this must always be done only when absolutely necessary, and with proper respect for the dignity of the detainee.
"It is simply inhumane to uproot somebody on the expectation that they will be returned to their home country only to then return them at the end of the day to a detention centre in the UK - sometimes a different one from the one they left that morning."
He went on: "The UK Border Agency must not wash its hands of responsibility for detainees just because the service is contracted out.
"The agency must introduce tougher management processes to ensure that contractors are delivering the service that the public expect, and that senior management challenge unacceptable behaviour."
Mr Mubenga's wife Adrienne Makenda Kambana said: "I am still waiting for justice.
"Nothing can bring my husband back now but the system must change to stop this happening to anyone else. I hope the Government will listen to what the committee has said and help others."
Deborah Coles, co-director of the charity Inquest, said: "We welcome parliamentary recognition that restraint during enforced removals is dangerous, unauthorised and potentially lethal.
"That this committee has condemned the appalling procedures and racist culture surrounding these removals once again highlights the lack of accountability of UKBA and their private contractors, as has been previously documented by a number of NGOs."
She went on: "That a culture of secrecy pervades the use of force on detainees is underlined by the refusals of UKBA and the Home Office to release the guidance on the use of force and restraint provided to escorting contractors.
"Few changes appear to have been made after the death of Jimmy Mubenga. Surely this must now prompt the Government into decisive action."
A G4S spokeswoman said: "G4S is no longer involved in the detention and escorting of foreign nationals from the UK.
"Matters relating to the death of Mr Mubenga are currently the subject of a police investigation and as such we are unable to comment."
A UKBA spokesman said: "All escorts on flights are trained to use restraint techniques approved by the Prison Service.
"We have no evidence contractors use head-down restraints and would take strong action against them if their conduct does not meet our standards.
"We are currently working closely with Noms (National Offender Management Service) to develop specific techniques appropriate to confined spaces."